1. Field of the Invention
The invention relates to handoff of mobile stations within cellular radio systems and, more particularly, to handoff within layered cell structures of such systems.
2. History of the Related Art
In cellular radio communication systems a geographic area is divided into a plurality of individual regions referred to as cells, each of which is provided with radio service on a plurality of separate RF channels the frequencies of which are reused among different cells sufficiently separated from one another that there is a sufficiently low level of interference between them. When a mobile station receiving radio service from a particular base station serving a particular cell moves from that cell into another adjacent cell, communication with the mobile station is "handed off" from the first base station to the second base station serving the adjacent cell. Such handoff is accomplished by a mobile switching center (MSC) to which all of the base stations are connected and which controls the allocation of communication channels to the individual mobiles moving throughout the region served by the cells.
As the demand for cellular radio service increases over time, the capacity of existing systems has been severely stressed to serve all the subscribers who would like to have access to the system, particularly in major metropolitan areas. To address this demand, cellular radio technology is currently moving from analog based systems, in which each subscriber communication channel is allocated to a single radio channel, to digital based systems in which a plurality of subscriber channels can be assigned to each radio channel through time division multiple access (TDMA) radio technology. In TDMA radio, each radio channel is divided into a plurality of time slots and a digitized portion of each subscriber's conversation is broadcast in an assigned time slot. However, even with such improvements in channel capacity, there exist certain areas within major metropolitan areas in which the demands on the system are so great that it cannot be successfully satisfied by existing cellular radio architectures. For example, in the area in and around a convention center located in a major metropolitan area, the channel usage by portable cellular radio transceivers may be so great that the demands for service cannot be satisfied by the entire channel capacity of the base station serving the cell within which the convention center is located. In such situations, it has been proposed to provide additional "layers" of cellular radio coverage provided by additional lower powered base stations located within an existing, so-called "umbrella" cell and referred to as "microcells" Such microcells may have a coverage or service area on the order of a few hundred meters in contrast to a few kilometers of coverage by the base station of the overlying umbrella cell. A plurality of such microcells may be located adjacent to one another and form a contiguous coverage area of substantial width all of which is within the overall coverage area of the umbrella cell.
When a layered cell structure, as described above in conjunction with umbrella cells and microcells, is used there is provided an enhanced level of radio capacity which can be configured for individual circumstances and which provides an assurance that users can receive service despite an extremely high demand within a very small geographic area. Moreover, additional layers of radio coverage may be added, for example, by a plurality of either contiguous or separated "pico cells" positioned within the coverage or service area of the individual microcells, each of which are in turn within the overall umbrella cell. The base stations providing the radio coverage within the pico cells would be of even still lower power than the base stations serving the microcells and have a coverage or service area of, for example, a hundred meters to provide coverage within a single building or a single floor within a large convention center.
Thus, in layered cell architectures, the issue of server selection and handoff of each mobile radio transceiver moving within a geographic area involves many more options. That is, it is possible for the mobile station to receive radio service at any given moment from either a pico cell base station, a microcell base station, or an umbrella cell base station. When conventional handoff criteria used in single layered cellular architectures are applied to this situation, problems arise and the solution is less than ideal. The ability to configure the handoff arrangement for maximum efficiency with respect to the utilization of channel availability and consistent with high quality radio service to each mobile subscriber is highly desirable.
When handoff is effected between adjacent cells in a single layer cellular radio architecture, the principle criterion used is the quality of the signal received from the mobile station by the respective base stations capable of providing radio service. That is, the quality of the signal received from the mobile station by the base station currently serving the mobile is compared with the quality of signal received by a base station serving an adjacent cell and when the quality of signal in the latter exceeds the former, the mobile is handed off to the base station serving the adjacent cell. In addition, a signal quality increment, known as an offset or hysteresis, is also applied to the signal quality difference value so that unless the signal quality in the adjacent base station is at least "x" amount greater than the presently serving base station, handoff does not occur. This prevents oscillating handoffs due to signal quality perturbations in which the mobile is repeatedly handed back and forth between two adjacent base stations.
When conventional handoff techniques, based purely upon signal quality, are applied to the multiple layer cellular architecture, the resulting service quality is often not optimal and in many cases totally inappropriate. This is because it is generally preferable to serve a mobile station with the lowest possible level of base station as a preferred service provider primarily because of capacity reasons. That is, if sufficient signal quality exists from the microcell, it is preferred to serve the mobile from that cell rather than handing the mobile off to the umbrella cell which has fewer total channels available for service than a plurality of adjacent microcells would have.
Because of the foregoing problems associated with the use of existing handoff algorithms within layered cellular architectures, it is desirable to introduce a system of handoff algorithms which maximize the efficient utilization of channel availability within a multi-level cellular radio architecture. The system of the present invention provides such a technique.